1. Field of the Invention
This invention relates to thermally bonded products and, more particularly, to thermally bonded fibrous products of a novel and advantageous composition suited for use as diesel soot filters, kiln furniture, combustor liners, burner tubes and other rigid high temperature insulation applications.
2. Description of the Prior Art
Ceramic fibers are often used to provide thermal insulation from very high temperatures. For many applications, it is convenient and often necessary to form the fibers into rigid shapes such as boards or specialty configurations. Two approaches are used to accomplish bonding of ceramic fibers to obtain these shapes. The first approach, chemical bonding, is the most common. A heat or air setting binder, usually in the liquid form, is added to the fiber and through drying, the application of low to moderate heat, or chemical reaction, the binder becomes rigid and literally glues the fibers into a rigid structure. Such binders include colloidal oxide suspensions, silicate solutions and thermoplastic suspensions. The strength of these systems are relatively low, typically having 20 to 50 psi flexural strength properties. More binder can be added to increase the strength. However, the additional binder adversely affects thermal insulating properties.
The chemical bonding method of producing a fibrous product is primarily used for low cost applications when no post drying heat treatment is conducted as part of the binding process. The result of this composition is a product which loses much of its strength and integrity after subsequent heat treatment. This limits the product's usefulness in applications in which handleability or load bearing capacity are important once the product has been subjected to temperatures over 1800.degree. F.
A second approach has been developed which utilizes rigid ceramic fiber shapes made of very expensive raw materials and is subjected to a sintering treatment after forming and drying. These materials are lightweight insulation materials targeted for use on reusable space shuttle vehicles. The success of these materials, as taught in U.S. Pat. No. 4,148,962, depends on the use of very pure raw materials as well as highly controlled processing, such that foreign contaminants are excluded from the finished fiber product.
The strengths of these types of products are normally 10 to 20 times that of fibrous products of the type produced by the chemical bonding approach. These products also retain their properties after heating above the 1800.degree. F. range. A major property desired as part of these products is the resistance to devitrification during subsequent heat treating. This cristobalite is subject to disruptive phase changes and would therefore, be unsuitable for the intended use, space shuttle tiles. It has been found that by using high purity materials, the devitrification of the glassy components can be minimized and kept within the acceptable range.
The requirements for successful application of this second approach to the production of fibrous insulating products teach against the use of lower grades of fibers and/or lower grade particulates. Such an approach would be suitable for use in applications which are identified as requiring the present invention, were it not for the high cost of the materials. The level of purity and degree of process control required result in the product costs associated with the second approach prohibit its widespread acceptance and use. Furthermore, the applications targeted for the present invention do not require the strict resistance to devitrification that is a specific goal of this second approach to the production of ceramic fiber shapes.